In a conventional speaker, a sound propagates in form of a dilatational wave (a sound wave/longitudinal wave), thereby stimulating human's hearing sense. A speaker which is different from such a conventional speaker is disclosed in Japanese Laid Open Patent Nos. 2000-350285, 2006-207108, and 2006-197562.
Japanese Laid Open Patent No. 2000-350285 discloses a speaker having a diaphragm, a support member which supports the diaphragm in a state where internal stress is produced therein, and a sound source connected with the support member.
Japanese Laid Open Patent No. 2006-207108 discloses a receiver, a piezoelectric device which vibrates based on an audio signal received by the receiver, and a helmet with a speaker to which the piezoelectric device is attached. Moreover, a sheet-like speaker to which the piezoelectric device is brought into close contact with a flexible sheet is disclosed.
Japanese Laid Open Patent No. 2006-197562 discloses a speaker having a piezoelectric device which vibrates based on an input signal, and a wave motion radiation unit which is brought into close contact with an oscillating face of the piezoelectric device.
In Japanese Laid Open Patent Nos. 2006-197562 and 2006-207108, physical characteristics of such a speaker, which are different from those of the conventional speaker are described as set forth below.
That is, the speaker uses the wave motion property of sound. Since the sound from the speaker may propagate in the air as a wave motion, energy loss and its attenuation rate are low, as compared with the conventional speaker. The sound quality and a propagation property thereof are excellent.
In addition, from another point of view, the sound from the speaker described above may be deemed to propagate in the air as particles. However, the mechanism of such a physical phenomenon of the speaker is not necessarily clearly explained.